There are two kinds of observational studies: (1) case control and (2) cohort. These studies approach a problem from different directions. Case control studies start with cases of disease (or injury or health) and compare them with controls without the disease in terms of possible causal factors. Cohort studies start with healthy people and follow them up to see what factors in their lifestyle or occupation or birth weight or school achievement, etc. are later related to the occurrence of disease, injury or better heath.